1. Field of the Invention
Embodiments of the present invention relate to helicopter systems and methods that enable a blade of a rotor of a helicopter to generate higher lift with less drag and to delay stall at high Mach numbers. More particularly, embodiments of the present invention relate to main rotor helicopter blades, helicopter systems, and methods for designing helicopter blades that include or create aft camber, which generates higher lift with less drag and delays stall at high Mach numbers.
2. Background Information
In horizontal flight, a blade of the main rotor of a helicopter acts as a moving airfoil or wing. Unlike the wing of an airplane, however, the main rotor blade of a helicopter is periodically moving in the direction of movement of the helicopter and periodically moving in a direction opposite to the direction of movement of the helicopter. Therefore, a main rotor blade of a helicopter experiences two vastly difference airspeeds in a very short period of time.
For example, if the main rotor of a helicopter is rotating at 700 feet per second and the helicopter is moving forward at 400 feet per second, a blade moving forward is experiencing an airspeed of 1,100 feet per second. When the blade is moving backward, however, it is experiencing an airspeed of only 300 feet per second. This vast change in air speed can occur in as little as two tenths of second.
The lift coefficient of a helicopter blade is a constant and is proportional to the lift divided by the square of the airspeed. Since the airspeed experienced by a blade on the backward moving side is much less than the airspeed experienced on the forward side, the lift on the backward side is much less than the lift on the forward side, creating an imbalance. One method of increasing the lift on the backward side is to increase the angle of attack of the blade on the backward side.
As the forward speed of a helicopter is increased, the angle of attack of the blade on the backward side must be increased to increase lift. At some point, the angle of attack is too great and the blade stalls. When the blade stalls, it can no longer provide increasing lift. The angle of attack at which the blade stalls defines a maximum lift capability of the blade and provides a limitation on the forward speed that can be achieved.
Another limitation on the forward speed of a helicopter is drag. If at the same time the lift of a backward moving blade of main rotor of helicopter is increased the drag is also increased significantly, the speed available due to the increased lift will be limited by the corresponding drag. As a result, it is desirable to increase the lift of a helicopter blade while maintaining the lowest possible lift to drag ratio.
The forward speed of a helicopter is also affected by the tip speed of the blades of the main rotor. The speed of a helicopter blade increases with radius so that the tip of the blade achieves the highest speed. The tip of a blade can achieve speeds approaching the speed of sound. Speeds close to the speed of sound are referred to as high Mach numbers.
As a helicopter blade reaches high Mach numbers, the blade begins to stall. The stalling of the blade, again, limits the forward speed the helicopter can achieve. As a result, it is desirable to delay the stall of the main rotor blade of a helicopter as the blade experiences a high Mach number.
In view of the foregoing, it can be appreciated that a substantial need exists for systems and methods that can advantageously provide helicopter blades with higher lift with less drag and that can delay stall at high Mach numbers.